The complexometric behavior of selected aroyl-S,N-ketene acetals shows that they are more than AIEgens

Using the established synthetic methods, aroyl-S,N-ketene acetals and subsequent bi- and multichromophores can be readily synthesized. Aside from pronounced AIE (aggregation induced emission) properties, these selected examples possess distinct complexometric behavior for various metals purely based on the underlying structural motifs. This affects the fluorescence properties of the materials which can be readily exploited for metal ion detection and for the formation of different metal-aroyl-S,N-ketene acetal complexes that were confirmed by Job plot analysis. In particular, gold(I), iron(III), and ruthenium (III) ions reveal complexation enhanced or quenched emission. For most dyes, weakly coodinating complexes were observed, only in case of a phenanthroline aroyl-S,N-ketene acetal multichromophore, measurements indicate the formation of a strongly coordinating complex. For this multichromophore, the complexation results in a loss of fluorescence intensity whereas for dimethylamino-aroyl-S,N-ketene acetals and bipyridine bichromophores, the observed quantum yield is nearly tripled upon complexation. Even if no stable complexes are formed, changes in absorption and emission properties allow for a simple ion detection.

and sulfur atom of the benzothiazole of the 2,5-bis[3-benzyl-2-methylbenzothiazole] croconaine derivative in comparison to the structural motif of the aroyl-S,N-ketene acetals, these complexing properties inspired to perform comparable complexometric studies for the aroyl-S,N-ketene acetal class of compounds.Due to the emission properties of the dimethylamino-substituted aroyl-S,N-ketene acetals and their derivates, we chose to make them the centerpiece of the present complexometric investigations.

Synthesis
First generation dimethylamino aroyl-S,N-ketene acetals 3a 54 and 3b 54 can be easily obtained via an addition-elimination sequence, starting from the respective acid chloride and a benzothiazolium salt 2 using diisopropylethylamine as a base in very good yields (Fig. 1A) 54 .
Another fluorescent aroyl-S,N-ketene acetal based material, which might be suitable for complexometric studies, are triphenylamine aroyl-S,N-ketene acetals that are accessible via a one-pot procedure, where the addition-elimination procedure is followed en route by a Suzuki coupling with triarylamine boronate 4a 65 to give the respective bichromophore 5a 57 in a yield of 66% (Fig. 1B) 57 .Similarly, a single step Suzuki reaction of dimethylamino aroyl-S,N-ketene acetal 3a and bipyridine boronic acid ester 4b leads to the formation of the respective bipyridine bichromophore 5b in excellent yield (Fig. 1C).

Photophysical properties
The photophysical properties of the investigated series of dyes in ethanol are mostly determined by the aroyl-S,Nketene acetal moiety.As most of the dyes comprise of dimethylamino aroyl-S,N-ketene acetals, for compounds 3a, 3b, and 5b show nearly identical absorption properties with absorption maxima of 404 nm, only bichromophore 5a and multichromophore 5c deviate from this behavior due to the secondary chromophores of triphenylamine and 1,10-phenanthroline, resulting in absorption maxima of 373 nm.Regarding the luminescence in ethanol, the emission properties are determined by the dimethylamino aroyl-S,N-ketene acetal units due to their intense emission at 455 nm and fluorescence quantum yields Φ f of around 0.20.Only bichromophore 5a fluoresces at 447 nm as the unsubstituted aroyl-S,N-ketene acetal 3c does not fluoresce in solution 54 .Therefore, the properties are determined by the secondary triphenylamine chromophore (Fig. 2A).The solid-state emission exhibits a greater range, which is determined by both the aroyl-S,N-ketene acetal and the secondary chromophore.The most blue shifted derivative is the triphenylamine bichromophore 5a with a solid-state emission maximum of 500 nm, whereas the most bathochromically shifted emission maxima occur for both the dimethylamino bipyridine bichromophore 5b and the brominated dimethylamino aroyl-S,N-ketene acetal 3a with λ max,em = 604 nm (Fig. 2B) (for full details of photophysical properties, see SI chpt.6).
As aggregation induced emission is normally a prime feature for aroyl-S,N-ketene acetals, we could only observe partial aggregation caused quenching effect in conjunction with a bathochromic shift of the emission maximum, which can be unequivocally attributed to the dimethylamino substituted aroyl-S,N-ketene acetal core (for details, see SI, chpt.7).

Complexometric studies
The standard method for determining complex stoichiometry is to use the method of continuous variation that is shown in the Job plot 67,68 .This assumes a constant total volume and concentration of all components involved in the equilibrium.The continuous variation of the ratios of metal salt and ligand concentration allows the complex stoichiometry to be determined via the change in the physical quantity under consideration.For the required Job plot, the variable photophysical quantity must be plotted against the mole fraction.It should also be noted that free binding sites in the complex can be occupied by solvent molecules if coordinating solvents are present 67,68 .
A series of qualitative tests were carried out for the dimethylamino-substituted compound 3b by adding small amounts of various metal salts to a solution of the dye in ethanol and observing with the naked eye the influence this has on the absorption and emission properties of the dissolved chromophore.As expected, gold(I), iron(III), and ruthenium(III) salts were identified as potential candidates for complexometric studies.We screened for a variety of the most common metal salts of both transition metals and metals like sodium, potassium, magnesium, calcium, aluminum, silver, gold, iron, tin, nickel, platin, zinc and scandium ions in order to cover up the widest range possible of metals that are most commonly used and can be called ubiquitous.
First, the complexation properties of the aroyl-S,N-ketene acetal 3b with gold(I) iodide are investigated (Fig. 3).This initially revealed an interesting effect when looking at the emission properties during complexation: there is a continuous increase in the emission intensity and an associated increase from Φ f = 0.20 in the absence of a metal salt to Φ f = 0.55 in presence of AuI (2.43 equivs) (Fig. 3B).A further increase of the amount of metal salt causes a slight decrease in the emission intensity.
The formation of difference spectra before and after addition of the metal salt reveals the most pronounced change for the absorption maximum at a wavelength of 271.5 nm (Fig. 3A).The corresponding absorbance values of the difference spectra at this maximum are plotted against the corresponding mole fraction χ of the ligand gives the respective Job plot that is fitted by a quadratic correlation with f(χ) = − 10.308 − χ 2 + 10.190 − χ − 2.1 29 and an excellent correlation factor (R 2 = 0.96).The paraboloid course of the function and the maximum of the negative parabola is localized at a mole fraction of χ = 0.5 indicates a weakly coordinating complex of aroyl-S,N-ketene acetal 3b and Au + ion.(Fig. 3C).
Several possible explanations for the observed fluorescence enhancements upon ligation can be given.The enhancement may result from ion-induced changes in the geometry or flexibility of the ligand.Other possibilities include chelation-induced changes of the relative energetic positions of different states, which shut off deactivation pathways as well as surpassing radiationless deactivation pathways via torsional motions which may be a viable explanation for aroyl-S,N-ketene acetals.The determination of the mechanism behind these fluorescence enhancements have been studied for more than 20 years and are still basis of currently ongoing investigations [69][70][71] .
Theoretically, various complexation modes of the dimethylamino-aroyl-S,N-ketene acetal 3b are conceivable.Coordination can either take place via the lone pairs of the dimethylamino group, the carbonyl oxygen, the www.nature.com/scientificreports/sulfur atom of the benzothiazole or simultaneously via the carbonyl oxygen and the sulfur atom, i.e. by chelation.
Previous studies suggested coordination via carbonyl oxygen and benzothiazole sulfur for 1:1 stoichiometry of comparable ligand systems 64 .In the case of the ruthenium complex, it can therefore be assumed that the ruthenium can be coordinated via carbonyl oxygen and benzothiazole sulfur as well as via the dimethylamino unit [72][73][74] .
For bichromophore system 5a, a pronounced effect of fluorescence quenching can be observed with the addition of iron(III) chloride; a weak effect can also be observed with regard to absorption spectroscopy.
In contrast to the complexometric studies of 3b (Fig. 4A), a decrease in fluorescence intensity of the bichromophore 5a can be observed with increasing mole fraction until the emission is finally extinguished (Fig. 4B).One possibility for the fluorescence quenching are competing electron transfer processes: the ligand is excited and Fe(III) is thus reduced to Fe(II) by the excited ligand.Another possible explanation addresses the effect of the comparable quenching behavior during protonation which we reported earlier for triphenylamine bichromophores 57 .The fluorescence can be attributed to the emissive triphenylamine secondary chromophore.However, an energy transfer to the aroyl-S,N-ketene acetal system can be assumed 57 .The systemic change in the molecule takes place at the aroyl-S,N-ketene acetal motif, which is responsible for the fluorescence quenching, and not at the triphenylamine, which is affected neither by complexation nor protonation.
The most obvious assumption of the complexation site is the sulfur and oxygen atoms of the aroyl unit and the benzothiazole unit.The determination of the difference spectra showed that the most pronounced effect could be observed for the absorption maximum at 271.5 nm.Therefore, this maximum was selected for the creation of the corresponding Job plot.
Plotting the mole fraction of the ligand against the relative absorbance at 271.5 nm results in a paraboloid curve, which indicates a weakly coordinating complex.The position of the maximum at χ = 0.5 indicates a 1:1 stoichiometry of the fluorescence quenching complex (Fig. 4C).
As for compound 3b, it was also possible to proceed for the combination of ruthenium(III) chloride and bichromophore 5b.With regard to the emission intensity, the intensity can also be increased by adding the dissolved metal salt.At the same time, the fluorescence quantum yield can be more than doubled from Φ f = 0.20 to 0.45 (Fig. 5B).In the case of ruthenium(III) chloride, the difference spectra reveal that the absorption maximum at 348.5 nm is most suitable for investigation using Job plot analysis (Fig. 5A).The Job plot of the mole fraction of the ligand χ against the relative absorbance at 348.5 nm provides a parabola that is highly correlated with the individual measurement points, so that a weakly coordinating complex can also be detected in this case.The maximum of this parabola was at χ = 0.5, meaning that a 1:1 stoichiometry of the ruthenium dye complex can be assumed (Fig. 5C).
Although no complex formation can be observed for other metal ions, the presence of various metal ions nevertheless has an obvious influence on the absorption and emission properties of bichromophore 5b dissolved in ethanol during qualitative tests.For this purpose, low concentrations of both the bichromophore 5b and the respective metal salt of c = 10 −5 m for absorption and c = 10 −7 m for emission are used.While an absorption maximum at 315 nm can be observed in the absence of a metal salt, this maximum disappears in the presence of metal ions.For the presence of gold ions, the maximum mentioned appears at 271.5 nm.The addition of ruthenium(III) chloride causes a bathochromic shift of the long wavelength absorption maximum.Furthermore, the addition of iron(II) sulfate, iron(III) chloride and aluminum(III) chloride results in a longer wavelength absorption maximum which can be ascribed to the long wavelength metal to ligand charge transfer bands.This absorption maximum occurs for Fe 2+ ions at 495 nm, for trivalent iron ions this maximum occurs with increased intensity bathochromically shifted to 505 nm.This effect can be observed even more strongly in the presence of Al 3+ ions, where the maximum occurs at 513 nm (Fig. 6A).With regard to the emission, an effect due to the presence of metal ions can also be demonstrated.Without the addition of metal salts, the emission maximum occurs at 447 nm.In presence of Pt 2+ ions and Ag + ions, a bathochromic shift of the emission maximum to 497 and 502 nm, respectively, can be observed.The most significant effect occurred in the presence of Al 3+ ions.The respective emission maximum can be detected here at 520 nm (Fig. 6B).
In comparison to the other systems 3b, 5a, and 5b investigated, a strongly altered emission behavior of the bis(dimethylamino-aroyl-S,N-keteneacetal)-1,10-phenanthroline system 5c can be observed.So far, the presence of the metal salts has led to a significant increase in the fluorescence quantum yield Φ f .In the case of the presence of the phenanthroline linker, the pronounced fluorescence quantum yield of the dimethylamino-aroyl-S,N-ketene acetals is already significantly reduced to 0.04.In addition, the fluorescence quantum yield Φ f decreases significantly with increasing mole fraction of the gold(I) iodide, with the greatest drop in emissivity already occurring upon addition of 0.11 equivs of gold(I) iodide.The further decrease in the fluorescence quantum yield Φ f with increasing mole fraction is almost linear (see SI, Fig. S15B).
The formation of the corresponding difference spectra reveals the greatest change for an absorption wavelength of 449 nm, so that these respective absorption values of the difference spectra at different mole fractions of the ligand could be used for the analysis using a Job plot (Fig. 7, top A).It is possible to fit the course of the complexation behavior by a quadratic function, so that formation of a weakly coordinating complex can be assumed.The maximum of the parabola is localized at a mole fraction of χ = 0.65, so that a complex consisting of two Au + ions and a molecule of the 1,10-phenanthroline system 5c is formed (see SI, Fig. S15C).
In contrast to previously investigated systems, complexation of Fe 2+ ions is observed in the case of derivative 5c, such as complexation must be attributable to the 1,10-phenanthroline linker.It can be seen that quenching of the emission in the presence of the metal salt is a material property of derivative 5c, since the initial fluorescence quantum yield Φ f of 0.04 decreases significantly when the metal salt loading is increased (Fig. 7, top B).The difference spectra of compound 5c reveal that the absorbance maximum at 400 nm is ideal for examination by Job plot analysis (Fig. 7, top right A).The plot of the absorbance at 400 nm against the mole fraction χ of the ligand again shows an almost parabolic curve and thus indicating a weakly coordinating complex.The position of the maximum at 0.6 suggests the formation of an iron-dye complex in a 1:2 stoichiometry (Fig. 7, top C).
Finally, the complexation behavior of the multichromophore 5c with respect to iron(III) chloride (Fig. 7, bottom A) will be discussed.The quenching of the emission reaches its maximum with this metal salt and chromophore combination.After addition of 0.11 equivs of iron(III) chloride, almost complete quenching of the emission intensity is observed.The fluorescence quantum yield Φ f decreases from 0.04 without any metal salt present to well below 0.01 upon the addition of the respective metal salt (Fig. 7, bottom B).With regard to the difference spectra obtained, the local maximum can be identified as suitable for Job plot analysis (Fig. 7, bottom A).The job plot of the mole fraction of the ligand χ against the relative absorbance at 362.5 nm yields a 7th degree function that is highly correlated with the individual measurement points and consists of two almost linear subsections, so that a strongly coordinating complex can be assumed in this case.At the same time, the position of the maximum of the 7th degree function at χ = 0.2 indicates the formation of a complex with a 5:1   www.nature.com/scientificreports/stoichiometry consisting of five ligands and one Fe 3+ ion 67,68 .This behavior can be interpreted that Fe 3+ ions coordinate to both 1,10-phenanthroline and the dimethylamino aroyl-S,N-ketene acetals (Fig. 7, bottom C).Even if no stable and observable metal dye complexes are formed, the presence of metal ions still have a considerable effect on both absorption and emission properties of dye 5c.To quantify the influence of the presence of the metal ions, low concentrations of c = 10 −5 m of both the metal salt used and the bridged system 5c were used for emission and concentrations of c = 10 −7 m were used.The intense absorption maximum at 274 nm disappears almost completely in the presence of various metal salts.For some metal salts, the absorption at 320 nm is only plotted, as interfering absorption effects of the metal salt solutions occur at higher energies.For platinum(II) salts and silver(I) iodide, the formation of an additional, energetically higher absorption maximum can be detected at 318 and 341 nm, respectively.The absorption maximum which can be ascribed to the aroyl-S,N-ketene acetal unit occurring at 373 nm in the absence of the metal salts occurs almost unchanged in the presence of Ag + , Fe 2+ and Al 3+ ions (Fig. 8A).
A similarly distinct behavior can also be detected for the emission of system 5c in the presence of various metal salts.In the absence of additives, the emission maximum in ethanol occurs at 455 nm.The emission is bathochromically shifted to 498 nm in the presence of Al 3+ ions, resulting in a greenish fluorescence.For the presence of Ni 2+ and Pt 2+ ions, the emission maximum in solution can be observed at 531 nm, whereas the emission intensity is clearly different and by a factor of 10 lower for the presence of Ni 2+ ions.Comparably, the emission maxima occur with the addition of Sn 2+ and Zn 2+ ions at 542 and 546 nm, respectively, with significantly broadened emission bands.The solution of chromophore 5c luminesces most intensely in the presence of silver(I) ions.Two emission maxima can be observed here, at 549 and 602 nm.The presence of Ru 3+ ions results in the most pronounced bathochromic shift of the emission maximum.The solution of the dye luminesces orange in ethanol with an emission maximum of 592 nm (Fig. 8B).
The discrepancy between the emission behavior of the derivatives 3a and 5b and the derivative 5c and their spectroscopically detected metal complexes suggests that two fundamentally divergent binding modes of the metal complexes are operative.It can be concluded that for chromophores 3a and 5b a constructive influence of the metal ions is in effect as emission quenching by energy or electron transfer does not occur in these cases.The most likely sites of coordination are in this case either the dimethylamino functionality or the potentially chelating oxygen-sulfur unit of aroyl and benzothiazole.For dimethylamino aroyl-S,N-ketene acetals, it is reasonable to assume that complexation also occurs via the amino nitrogen.But, as with all anilines, both the Brønsted and Lewis basicity is reduced by the aromatic compound, and even more so by the p-conjugation to the carbonyl group.Therefore, the carbonyl group even becomes more electron-rich as a result.Ergo, chelation, also for thermodynamic reasons, enthalpic and entropic, should actually be the most likely modus operandi for these systems.This means that the presence of the metal salt can have a fluorogenic effect as seen by the increase in the fluorescence quantum yield Φ f (Fig. 9, top) 23,[75][76][77] .A possible explanation for this behavior lies in the metal ion-induced changes in the geometry and flexibility of the ligand as well as reducing the accessibility of deactivation pathways of certain functional groups.Especially in case of flexible ligands like bipyridine or equally flexible aroyl-S,N-ketene acetal, the chelation process suppresses radiationless deactivation via torsional motions 40 .
The quenching of the emission in the presence of transition metal ions, as observed for 5c, clearly indicates that energy or electron transfer processes, as described in the literature 78,79 , lead to emission quenching.Decisive for such processes the corresponding metal must coordinate to the central 1,10-phenanthroline, which has been shown to act as a quencher 80,81 .Therefore, it can be assumed that a quenching energy transfer (EnT) process from the emissive dimethylamino aroyl-S,N-ketene acetals to the dark, non-emissive phenanthroline-metal complex can occur (Fig. 9, bottom) 76,77 .

Conclusion
In summary, bi-, multi-and single aroyl-S,N-ketene acetals were readily synthesized in mostly very good to excellent yields.Aside from typical AIE properties, these dyes exhibit pronounced complexometric behavior.Ruthenium, iron and gold cations form complexes with the investigated dyes as seen by photospectrometry.For most of the compounds, the dimethylamino substituent or the carbonyl oxygen and the sulfur of the benzothiazole are most likely sites of complexation resulting in a significant increase of quantum yield.In case of dimethylamino aroyl-S,N-ketene acetals and Au + -ions, the fluorescence quantum yield is nearly tripled upon complexation.Similar trends were also observed for iron and ruthenium ions and in case of dimethylamino aroyl-S,N-ketene acetal bipyridine bichromophores.In contrast, for the 1,10-phenanthroline bridged trichromophore, the quantum yield drops presumably due to a dark state of the complexed 1,10-phenanthroline unit.For most metal ion-dye combinations, only weakly coordinating complexes could be observed by spectroscopy, only for a phenanthroline aroyl-S,N-ketene acetal multichromophore in combination with Fe 3+ -ions, the formation of a strongly coordinating complex could be observed.In addition, even if no stable complexes are formed, the change of the emission color in the presence of different metal ions can be exploited for metal ion sensing.Future works are directed to address metal complexation for ion sensing and fishing, as well as to form more stable complexes.

Methods
The used chemicals which have not been synthesized were purchased at Acros Organics BVBA, Alfa Aeser GmbH & Co KG, Fluorochem Ltd., J&K Scientific Ltd., Merck KGaA, Macherey-Nagel GmbH & Co. KG, Sigma-Aldrich Chemie GmbH and VWR and have been used without further purification.The melting points have been measured with Melting Point B-540 of the company Büchi according to the protocol by Kofler 82 .All NMR and mass spectrometry experiments have been performed by the Heinrich Heine University Center of Molecular Structure Analytics (HHUCeMSA). 1 H, 13 C and DEPT 135-spectra have been measured at 298 K on an Avance III-300 and an Avance III-600 of the company Bruker.EI mass spectra have been measured with Triple Quadrupol spectrometer TSQ 7000 of the company Finnigan MAT.MALDI spectra have been measured with a MALDI/TOF UltrafleXtreme of the company Bruker Daltronik.IR spectra were recorded with neat compounds under attenuated total reflection (ATR) with IRAffinity-1 of the company Shimadzu.The elementary analyses have been measured with Perkin Elmer Series II Analyser 2400 or Vario Micro Cube of the company Analysensysteme GmbH.UV/Vis spectra of the dye solutions were measured with a Lambda 19 spectrometer from Perkin Elmer.The emission spectra of the dye solutions and the solid compounds were recorded with a Hitachi F-7000 spectrofluorometer using the emission correction curve provided by the instrument manufacturer.Emission spectra were not corrected for the wavelength-dependent spectral responsivity of the fluorometer.All solution spectra were recorded with dyes dissolved in spectroscopic grade solvents at 298 K using 1 cmquat z cuvettes from Hellma GmbH.The molar extinction coefficients of dye solutions of known dye concentration were determined by five-point regression line.www.nature.com/scientificreports/For aggregation studies, samples of the aroyl-S,N-ketene acetals were dissolved in various mixtures of organic solvents and water with water contents ranging from 0 to 99%, with ethanol/water mixtures giving the clearest results, which are discussed in detail below.Furthermore, solvent mixtures of cyclohexane and dichloromethane were also tested, but no aggregate formation was observed.A stock solution of the respective chromophore was prepared in ethanol.A defined amount of this stock solution was taken, and the corresponding amount of ethanol was first placed in a 10 mL volumetric flask before the corresponding amount of water was added.To exclude aging and coagulation processes of the formed aggregates as far as possible, all solutions were first treated in an ultrasonic bath for 5 min before the emission and absorption spectra were recorded starting with the solutions of the highest water content.Selected aggregate solutions were stored for investigation of aggregate stability over time for subsequent measurements at defined time points.For the photographs of the aggregate solutions, the corresponding measurement solutions were used directly.

Figure 9 .
Figure 9. Potential explanatory approach to rationalize the emission enhancement of 3a and 5a and the quenching of the emission of 5c in the presence of metal ions.